Objective Meta‐analyses show that nonbound ceruloplasmin (non‐Cp) copper (also known as free or labile copper) in serum is higher in patients with Alzheimer disease (AD). It differentiates subjects with mild cognitive impairment (MCI) from healthy controls. However, a longitudinal study on an MCI cohort has not yet been performed to assess the accuracy of non‐Cp copper for the prediction of conversion from MCI to AD during a long‐term follow‐up. Methods The study included 42 MCI converters and 99 stable MCI subjects. We assessed levels of copper, ceruloplasmin, non‐Cp copper, iron, transferrin, ferritin, and APOE genotype. A multiple Cox regression analysis—with age, sex, baseline Mini‐Mental State Examination, APOE4, iron, non‐Cp copper, transferrin, ferritin, hypercholesterolemia, and hypertension as covariates—was applied to predict the conversion from MCI to AD. Results Among the evaluated parameters, the only significant predictor of conversion to AD was non‐Cp copper (hazard ratio = 1.23, 95% confidence interval = 1.03–1.47, p = 0.022); for each additional micromole per liter unit (μmol/l) of non‐Cp copper, the hazard increased by ∼20%. Subjects with non‐Cp copper levels >1.6μmol/l had a hazard conversion rate (50% of conversion in 4 years) that was ∼3× higher than those with values ≤1.6μmol/l (<20% in 4 years). The rate of conversion was similar between APOE4 carriers and noncarriers (p = 0.321), indicating that the non‐Cp copper association was independent of APOE4. Interpretation Non‐Cp copper appears to predict conversion from MCI to AD. These results encourage healthy life style choices and dietary intervention to modify this risk. ANN NEUROL 2014;75:574–580